Internal combustion engines common to vehicles, such as automobiles, light trucks and sport utility vehicles, and, particularly those fueled by gasoline, inherently produce a loud and irritating roar through the engine exhaust during operation that, if not muffled, is unbearable to a person's ears. That noise generation becomes particularly loud and irritating when the gas pedal is quickly depressed, “floored”, to force the engine to rapidly accelerate to a high rpm. During the exhaust portion of the four-stroke engine cycle that follows detonation of the fuel and air mixture in the cylinder, the cylinder exhaust valve associated with an engine cylinder opens and the piston, being moved upwardly in the cylinder toward the exhaust valve, forces the gaseous products of combustion from the cylinder. Typical internal combustion engines contain multiple engine cylinders, four, six or eight cylinders, as example. Each cylinder in the engine is “fired” in serial order during the associated compression stage for the cylinder. Once fired, the resulting gaseous products of combustion are exhausted from the cylinder during the succeeding exhaust stage. The repetitive expulsion of the hot exhaust gases being forced from each engine cylinder in turn and rapid expansion of those gases into the exhaust manifold generates the engine noise. The hot exhaust gas empties into the exhaust manifold and thence flows into the exhaust runners, the metal tubes leading to the catalytic converter, and from the catalytic converter passes through the muffler, and empties from the muffler to the tailpipe and, thence, to the exterior atmosphere where both the gas is expelled and the sound is broadcast. With multiple engine cylinders, the foregoing exhaust action of engine operation produces a periodic series of gas pressure pulses and the repetition rate of those pulses varies as a function of the engine rpm. Typically, that pulse rate falls in the audible range.
Because of that noise, automobiles manufactured and/or sold are required to include a sound attenuating device, commonly referred to as a muffler, to dampen the exterior exhaust sound produced by the vehicle engine to at or below the intensity level specified by law. A typical exhaust muffler provided on the gasoline fueled automobiles of major automobile manufacturers, “the OEM muffler,” contains several perforated pipes housed within a closed chamber. One of those pipes, the inlet pipe, empties into a front chamber within the housing or casing and the second pipe provides an exit from a rear chamber. A resonator chamber located at the front of the housing is also coupled by a pipe or passage to the rear of the first chamber. Sound reduction in the muffler relies upon the sound cancellation produced by having reflected and direct portions of the exhaust gas pulse combine in opposite phase inside the muffler so that the sound released through the tailpipe is reduced in level.
Because the pulses of exhaust gas introduced into the muffler must pass through the inlet pipe and exit against a wall in the first chamber and thence return to the middle chamber, one effect of the presence of the wall is to produce a back-pressure at the inlet. Although the OEM muffler dampens the harsh sounds produced at the outlet of the tailpipe below the legal limit for sound, the obstruction created by the chamber wall inside the muffler housing produces a back pressure in the exhaust path from the manifold. To overcome the effect of that back pressure, the engine must perform extra work in pumping out the exhaust gas. In effect, the back pressure robs the engine of some amount of horsepower that could otherwise be obtained from the engine if the exhaust gas were exhausted directly to the atmosphere.
To reduce that back pressure and increase the available horsepower from the engine, performance mufflers were introduced as an after-market product to replace the OEM muffler. Serious performance aficionados could then replace the original equipment muffler with a performance muffler and achieve both better performance and a more desirable sound from the tailpipe.
Performance mufflers currently being marketed are designed to function by one of two basic techniques. One design incorporates fiberglass matting, a sound absorbent material on the outer walls of a perforated tube. The matting absorbs the sound of the resonant audio frequency produced by the exhaust gas as the exhaust gas moves through the perforations in the tube and dampens the sound to tolerable levels within the legal limit. Unfortunately, the matting often breaks down after prolonged use and is discharged into the tailpipe. The matting also absorbs oil and metallic minerals as may be included in the exhaust gases. The accumulation of those substances reduces the sound absorbency of the matting and, hence, the ability of the muffler to absorb or dampen the exhaust sound level. When that occurs, the muffler must be replaced.
The better performance mufflers rely on a chamber single deflector technology which does not require a packing of sound absorbent material. Instead the muffler permits the exhaust gases to flow through the muffler and exit the tail pipe more easily than the OEM packed muffler and produces a lower back pressure. The exhaust gases are directed in a path inside the muffler housing defined by internal metal baffles. Exhaust gas introduced into the performance muffler is directed through internal chambers to the right and the left of the muffler inlet. The foregoing path for the exhaust gas is less restrictive and permits the engine to develop greater horsepower than the absorbent packed muffler, while producing a deep throated rumbling sound desired by many as an advertisement of the power of their automobile engine, often called performance sound. Performance mufflers of the foregoing type have been available for some time from the Flowmaster Company of Santa Rosa, Calif. and variations of that muffler are described in U.S. Pat. Nos. 4,574,914, 4,809,812 and 5,123,502 to which the reader may make reference.
The adaptation of emission controls on automobile internal combustion engines resulted in making combustion more efficient and lowered exhaust gas temperatures and catalytic converters were included in the routing of the exhaust gas, all of which aids the effectiveness and/or reliability of an exhaust gas muffler. However, although of aid, those additional systems are not for the purpose of muffling engine noise at the exterior below the sound limit and do not do so.
Although solving the problem of exterior noise as might be experienced by a bystander to the vehicle, the muffler should also minimize the engine noise that reaches the interior of the automobile and could be disturbing to the automobile owner, in practice, one finds that OEM mufflers and performance mufflers don't always provide appropriate muffling under all driving conditions. As example, it is found that the internal combustion engine of many sport utility vehicle produces a sound in the interior of the vehicle that is discomforting, if not irritating, that occurs when the engine is operating at about 2200 rpm, which typically corresponds to driving the automobile at a speed of about sixty miles per hour, a typical cruising speed. The engine also produces that annoying sound on acceleration as the engine passes through the 2200 rpm speed. Though the muffler achieves sufficient quietude at other speeds, it appears to produce or allow a resonance inside the vehicle cabin at the 2200 rpm engine speed, which is obviously undesirable.
Then too, when the engine is operating at a high speed above 2200 rpm and the driver removes his foot from the accelerator pedal to allow the vehicle to decelerate, an annoying crackling or “popping” sound is produced inside the cabin that originates at the muffler. That sound is disconcerting to most drivers who may think an engine backfire is imminent. Small pick-up trucks experience a similar problem with cabin sound that the muffler fails to handle when the truck is placed under a heavy load, such as when towing a camper or recreational vehicle, horse trailer or the like.
Muffler durability is also a problem. One finds that some performance mufflers develop hot spots on the muffler case during engine operation. Sometimes the intensity of a hot spot is so great as to produce through localized thermal expansion a bulge in the side of the metal muffler case. That thermal action is likely to lead to a break through in the side of the muffler through which exhaust gases and sound escapes to the exterior. Should that occur, the muffler must be replaced. The foregoing hot spots appear to inherently result from the effect of the baffles located inside the performance muffler, earlier noted. Apparently, a portion of the exhaust gas passing through the muffler is diverted by the internal baffles to create localized vortexes of hot gases in the interior of the muffler. Those vortexes remain stationary in location and don't readily exit the muffler, producing steady heating at a spot on the side of the muffler that, like a blowtorch, ultimately burns through the metal of the muffler case. As an advantage, the present invention avoids such burn-through.
Even before any burn-through occurs, the very high temperatures produced by such hot spots in the performance muffler often results in driver discomfort or increased fuel consumption. Located on the undercarriage of the vehicle the heat from the muffler is conducted or convected in some measure through the vehicle flooring to the interior of the automobile, which, in the summer, is discomforting to the driver, if automobile air conditioning is unavailable. If air conditioning is available, prolonged operation of the air conditioner is necessary to dissipate the accumulating heat and maintain a comfortable cabin temperature. But prolonged operation of the air conditioner results in greater gasoline consumption, lowering overall engine efficiency. As an advantage, the present invention avoids heating the interior of the vehicle to such a degree.
The OEM mufflers are principally designed to muffle sound. Performance mufflers, on the other hand, are designed to reduce the intensity of the exhaust sound and also produce a satisfying sound of low frequency and timbre characteristic of performance vehicles. That sound is sometimes referred to by auto enthusiasts, including the present inventor, as a performance sound. Psychologically, the performance sound gives an audible clue that the vehicle contains great horsepower. Difficult to describe with words and lacking precise definition, the sound may be said to be one that one knows when one hears the sound. As an advantage, the present invention also delivers performance sound.
The foregoing difficulties were noted in exhaust gas mufflers used for gasoline fueled internal combustion engines. Many light trucks and some automobiles today instead use diesel fueled internal combustion engines. Operated without a muffler, the sound generated during operation of the diesel engine is typically of an acceptable frequency or timbre, but the sound produced is also uncomfortably loud and must also be muffled. As a further advantage, the present invention can be employed with diesel engines.
Accordingly, an object of the present invention is to muffle the sound of the exhaust gas exhausted from an internal combustion engine and instead permit generation of a performance sound.
An additional object of the present invention is to minimize or prevent the appearance of annoying engine exhaust sounds in the vehicle interior over the normal range of engine speed, while limiting the sound appearing exterior of the vehicle below the legal limit of loudness and providing a performance sound.
A further object of the invention is to prevent the appearance of annoying sounds within the driver's compartment of a vehicle during normal driving speeds and under heavy loads while maintaining exterior sound of sufficiently low level.
And, an ancillary object of the invention is to improve the thermal stability and operational reliability of the exhaust gas muffler for an internal combustion engine, and, more specifically, provide a muffler structure that does not develop hot spots in the casing wall of the muffler during engine operation.